A subdomain is the part of the web address which is before a domain and you've quite possibly seen a lot of subdomains while browsing the Internet. For example, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of employing a subdomain is that it can have a separate website and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary website, such as an e-learning portal for students in addition to the primary school website. If you use subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more secure to have the sites separate from one another.